To drive drill bits, it is known to use downhole roller vane motors. These motors are driven by the drilling mud that is pumped down through the drill string to lubricate and cool the bit and to carry drill cuttings back to the ground surface through the annular space between the drill string and the borehole wall.
Roller vane motors with inner and outer housing and with the inlet/outlet ports in the inner housing are described in WO 93/08374. Roller vane motors with combined inner and outer housing, with inlet ports in the rotor and outlet ports in the housing are described in WO 94/16198.
In the above motors, rollers that are located in the extended position in recesses in the rotor are pushed by the drilling mud in chambers between rotor and (inner) housing from inlet ports towards outlet ports in a clockwise direction. Rollers that are not pushed by the drilling mud towards an outlet port are not subjected to the mud pressure since they have been forced into a retracted position by longitudinally extending wing deflector cams along the inner wall surface of the (inner) housing.
Advantages of the known roller vane motor with combined inner and outer housing compared to the roller vane motor with both inner and outer housing are its simpler construction and the greater torque per unit length of the motor.
A drawback of the known roller vane motor with combined inner and outer housing is that the pressure drop across the motor must be equal to the pressure drop across the drill bit, since these pressure drops are parallel. Moreover, the flowrate of the drilling mud across the drill bit is reduced.